A compound waveguide contains a first, optical signal transmitting waveguide, and a second optical waveguide placed in optical proximity to at least a portion of the first waveguide. The second waveguide (possibly electro-optically tunable) can be used for the coupling of optical energy, within a selectable spectral band, to or from the first waveguide. The coupling effect between the first and second waveguides can be utilized in a variety of optical signal processing applications.
For example, in some filtering applications, efficient recollection of the coupled optical energy for further use (e.g., in signal detection or demodulation, source stabilization feedback) is required. Multiple wavelength bands may be transmitted through the first waveguide, in a wavelength division multiplexing (WDM) system, in which case the second waveguide can be used as a filter to extract information carried in one of the bands. If an electro-optic material is used to form the second waveguide, the device can be configured as an active, electro-optically tunable filter. In another embodiment, the geometric and physical properties of the second waveguide itself may result in a useful passive filter inherently tuned to a particular wavelength of interest.
As another example, if an electro-optic material is used to form the second waveguide, the device can be configured as an intensity modulator for a fixed wavelength signal transmitted through the first waveguide. By applying an electric field to the second waveguide using, for example, a suitable high-speed electrode pattern, a refractive index change can be induced in the second waveguide and a corresponding change in the spectral response results. The transmitted intensity of the signal in the first waveguide can therefore be modulated by the changing spectral response, resulting in a modulator which can operate at up to microwave frequencies.
The first, transmitting waveguide may be implemented as a fiber optic. There are at least two types of possible architectures for such fiber optic compound waveguides: one architecture in which the fiber is physically broken to accommodate the insertion of the second, coupling waveguide; and another architecture in which no fiber discontinuity is required. This second architecture (more fully discussed below) involves the removal of a portion of the cladding of the fiber optic and "overlaying" the second waveguide in optical proximity to this altered area of the fiber. Evanescent mode coupling therefore occurs between the fiber and the second, "overlay" waveguide as a function of the size, shape, and refractive index of the overlay waveguide.
This overlay waveguide architecture substantially confines the transmitted optical signal to the fiber core without significant core interruption, thereby providing low loss and high mechanical and thermal stability. When an electro-optic material is used as the overlay waveguide, the device can be used for electro-optic bandpass filtering or electro-optic amplitude modulation. (See, e.g., W. Jonstone, S. Murray, M. Gill, A. McDonach, G. Thursby, D. Moodie and B. Culshaw, "Fiber Optic Modulators Using Active Multimode Waveguide Overlays," Electron. Lett., 27 894 (1991), hereby incorporated by reference herein in its entirety.)
The criteria for high-performance electro-optic modulators are large electro-optic modulation bandwidth and low drive power. These criteria are dominated by the properties of the electro-optic materials employed, such as the electro-optic coefficient, dielectric constant, transparency, and linear refractive index. Inorganic ionic electro-optic crystals, such as LiNbO.sub.3 and KNbO.sub.3, have been well investigated and developed for such applications. In inorganic ionic crystals, lattice vibrations contribute significantly to the electro-optic coefficients and the dielectric constant leading to a strong frequency dependence of both properties. These ionic contributions increase electro-optic effects considerably but also increase the dielectric constants, thus limiting bandwidth. At high frequencies, the wavelength of the modulating electric field becomes shorter than the modulator length. In this case, modulation of the optical signal is achieved with traveling microwaves. There are limits to the modulation bandwidth due to a refractive index mismatch between the microwaves and optical waves. The 3 dB bandwidth of the modulator (frequency at which the power in the optical sidebands is reduced by one-half) is given by: ##EQU1## where n.sub.o and n.sub.m (=.sqroot..di-elect cons..sub.m ) are the refractive indices at optical and microwave frequencies, respectively, c is the speed of light, and L is the waveguide length. For example, for an LiNbO.sub.3 crystal, n.sub.o =2.2 (@632.8 nm), n.sub.m =4.2, the bandwidth .DELTA..function..sub.3 dB [GHz]=6.7 for L=1 cm; and for a Knbo.sub.3 crystal, n.sub.o =2.169(@632.8 nm), n.sub.m =4.9, the bandwidth .DELTA..function..sub.3 dB [Ghz]=4.9 for L=1 cm. Nevertheless, inorganic materials can be used for fast electro-optic modulators. By using special electrode/waveguide geometries, the microwave speed can be increased. For example, a 40 Ghz bandwidth Ti:LiNbO.sub.3 modulator has been demonstrated.
There are other limitations imposed on wide-band optical modulators for communication purposes based on fundamental physics. One of them is the power requirement in electro-optic amplitude modulators. The value n.sup.7 r.sup.2.sub.eff /.di-elect cons. can be regarded as a figure of merit for this configuration if a minimum drive power is required. For example, for LiNbO.sub.3 crystal, n=2.134 (@1300 nm), r.sub.33 =28 pm/V, .di-elect cons..sup.s =28, the figure of merit n.sup.7 r.sup.2.sub.e.function. /.di-elect cons. is 6.0[.times.10.sup.3 (pm/V).sup.2 ]; and for Knbo.sub.3 crystal, n=2.109 (@1300 nm), r.sub.33 =34 pm/V, .di-elect cons..sup.s =24, the figure of merit n.sup.7 r.sup.2.sub.eff /.di-elect cons. is 8.9 [.times.10.sup.3 (pm/V).sup.2 ].
The use of organic crystals has recently been proposed for optical coupling applications. (The term "organic crystal" as used herein is not meant to include any polymers that may have a crystalline structure.) Unlike the inorganic, ionic materials discussed above, the origin of the electro-optic effect in organic substances is mainly electronic, and therefore a smaller dependence on the frequency of the applied field is experienced. In addition, large electro-optic coefficients, high modulation bandwidths, and low drive powers are advantages of molecular crystals. 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate (hereinafter "DAST") is an organic salt crystal in which the stilbazolium, a very efficient organic chromophore, is used as the optically nonlinear part. A crystal structure analysis shows that the angular deviation of the charge-transfer axes of the stilbazolium chromophores is about 20.degree. from a complete alignment. Therefore, the exceptionally large nonlinear optical susceptibilities and the good alignment of the chromophores in the crystal indicate that DAST is a useful electro-optic material. As expected from the molecular arrangement, the electro-optic coefficients, r.sub.11, have been found to be quite large with values of 47.+-.8 pm/V at .lambda.=1535 nm, 50.+-.5 pm/V at .lambda.=1313 nm, and 77.+-.8 pm/V at .lambda.=800 nm, respectively. (See, e.g., F. Pan, G. Knopfle, Ch. Bosshard, S. Follonier, R. Spreiter, M. S. Wong, and P. Gunter, "Electro-Optic Properties of the Organic Salt 4-N,N-dimethylamino-4'-N'-methyl-stilbazolium tosylate," Appl. Phys. Lett. 69, 13-15 (1996), hereby incorporated by reference herein in its entirety.)
The high refractive index (n=2.18@1313 nm) and low dielectric constant (e.g., .di-elect cons..sub.11 =5.2) are additional advantages for high-speed electro-optic modulation. At .lambda.=1313 nm, which is a wavelength appropriate for optical communication, DAST crystals offer a bandwidth .DELTA.f.sub.3 dB of 140 Ghz for L=1 cm, and their figure of merit n.sup.7 r.sup.2.sub.eff /.di-elect cons. is 130[.times.10.sup.3 (pm/V).sup.2 ]. (See, e.g., Ch. Bosshard, M.-S. Wong, F. Pan, R. Spreiter, S. Follonier, U. Meier and P. Guinter, "Novel Organic Crystals For Nonlinear and Electro-Optics" R. W. Munn et al (Eds.) in Electrical and Related Properties of Organic Solids, 279-296 (1997), hereby incorporated by reference herein in its entirety.) The bandwidth and figure of merit of DAST are about 21 times larger than those of LiNbO.sub.3. Their large electro-optic coefficients, high modulation bandwidths, and low power consumption make DAST crystals highly desirable for electro-optic applications.
What is now required are improved, easily produced organic crystal compound waveguide configurations, and reliable methods for their production, which will offer the required levels and quality of optical coupling to and/or from the optically proximate transmitting waveguide, while maintaining the advantages of the crystals' large electro-optic coefficients, high modulation bandwidths, and lower power consumption.